The present invention relates to a waste water treatment method and waste water treatment equipment for concurrently treating in one tank fluorine, nitrogen, organic matter and the like in waste water discharged from a semiconductor plant or a liquid crystal plant.
In a semiconductor plant or a liquid crystal plant, a large amount of organic matter such as IPA (isopropyl alcohol) and acetone as well as hydrofluoric acid including fluorine together with nitric acid and ammonia water including nitrogen. There is also used ammonium fluoride as a compound of fluorine and nitrogen.
From the point of view of the water pollution control law, it is required to treat the organic matter that increases the fluorine content, nitrogen content and the COD (Chemical Oxygen Demand) to a specified concentration in the case of a waste water that contains nitrogen and organic matter in addition to fluorine. Depending on the location of the plant, a severer additional standard is provided, and accordingly, a severer treatment is sometimes performed in conformity to the above standard and the standard owned by each individual enterprise.
In this case, defluorination is necessary for satisfying the regulation value of the fluorine concentration by the law, the additional standard of administration and the in-house standard of each enterprise. Further, denitrification is also necessary for satisfying the regulation value of the total nitrogen concentration in a waste water, the additional standard of administration and the in-house standard of each enterprise. The removal of the organic matter is necessary for reducing the COD that serves as a regulation value of waste water.
Conventionally, in a semiconductor plant, a mixed waste water having pH 2 to pH 3 containing nitrogen and organic matter in addition to fluorine has been treated as follows.
That is, as shown in FIG. 11, first, fluorine is chemically treated to be neutralized by chemicals such as slaked lime in a coagulo-sedimentation process 32. After the fluorine is neutralized, then nitrogen is treated (nitrified) to nitrate nitrogen while biologically treating the organic matter by aerobic microorganisms in a nitrification tank 33. Subsequently, an organic matter that serves as a hydrogen donor is excessively added from an organic matter tank 26 to a denitrification tank 29 so as to treat the nitrate nitrogen as nitrogen gas dissipating into the atmospheric air. Next, the organic matter excessively added to the denitrification tank 29 is biologically treated in a re-aeration tank 31.
As defluorination equipment for removing fluorine in waste water, there is the one shown in FIG. 12. In this defluorination equipment, a fluorine containing waste water is put through two calcium carbonate containing tanks 101 and 102, and the water flowing from the calcium carbonate containing tank 102 is introduced into a circulation tank 103. Then, the waste water inside the circulation tank 103 is introduced into a membrane filter unit 104, which separates the waste water into a concentrated water including calcium carbonate crystals flowing out of the last calcium carbonate containing tank 102 and a permeated water. Subsequently, the thus-separated concentrated water is sent back to the circulation tank 103. Part of the concentrated water is sent back to the first calcium carbonate containing tank 101. The permeated water is discharged into a reservoir tank 105.
According to the above defluorination equipment, the fluorine in the waste water reacts with the calcium carbonate located inside the calcium carbonate containing tanks 101 and 102 and becomes calcium fluoride. Then, after a lapse of a specified time of treatment, the calcium fluoride is extracted from the calcium carbonate containing tanks 101 and 102. Thus, the fluorine is removed in the form of calcium fluoride.
As another defluorination equipment, there is calcium fluoride collecting equipment as shown in FIG. 13. In this calcium fluoride collecting equipment, calcium carbonate located inside a calcium carbonate silo 107 is added to a fluorine containing solution located inside a calcium carbonate reaction tank 106. Then, through a high-temperature aerating process (high-temperature decompression deaerating process) at a temperature of 50.degree. C. to 100.degree. C., calcium fluoride is collected. It is to be noted that the reference numeral 108 denotes a blower for the aeration and the reference numeral 109 denotes an air diffusion pipe.
The pieces of equipment shown in FIG. 12 and FIG. 13 can also treat fluorine, but they cannot treat the organic matter.
In view of the above, there is waste water treatment equipment for treating a fluorine waste water containing organic matter, as shown in FIG. 14. This waste water treatment equipment removes both fluorine and organic matter in the waste water by skillfully utilizing chemical reaction and biotic reaction with aerobic microorganisms by, for example, making calcium carbonate mineral 124a, which serves as a filler, flow strongly and weakly.
In FIG. 14 are shown a first water tank 111 for executing a principal treatment, a second water tank (sedimentation tank) 112, a third water tank (sludge concentration tank) 113, a polychlorinated aluminum tank 114, clarifiers 115 and 116, blowers 117 through 119, a line mixer 120, a diffuser 121 and air diffusion pipes 122 and 123. There are further shown calcium carbonate mineral pieces 124a through 124c, an inorganic sludge 125 and microorganic sludge 126.
FIG. 15 shows another type of waste water treatment equipment. This waste water treatment equipment is to treat fluorine waste water containing organic matter by using calcium carbonate mineral. This waste water treatment equipment uses all the fillers in a fixed state. Therefore, the calcium fluoride generated as a consequence of the reaction of fluorine in the waste water with calcium remains between the calcium carbonate mineral pieces 146 in the fixed state and exists as a mass for a long time. Then, the mass gradually becomes larger to spread throughout the inside of the whole tank, as a consequence of which the treatment efficiency reduces. In FIG. 15 are shown a first reaction regulation tank 131, a second reaction regulation tank 132, a third water tank 133 that serves as a reaction coagulation tank, a fourth water tank 134 that serves as a sedimentation tank, a fifth water tank 135 that serves as a sludge concentration tank, a filter press 136, water sprinkling tanks 137 and 138, blowers 139 through 141, air diffusion pipes 142 and 143 and clarifiers 144 and 145. There are further shown calcium carbonate mineral 146, a charcoal 147 and plastic filler 148.
The pieces of equipment shown in FIG. 14 and FIG. 15 can treat fluorine and organic matter, but they cannot treat nitrogen.
In view of the above, there can be considered equipment as shown in FIG. 16 besides the equipment shown in FIG. 11 as equipment capable of treating fluorine, organic matter and nitrogen. This waste water treatment equipment is intended for treating fluorine waste water containing nitrogen and organic matter. This equipment treats the organic matter and fluorine in the waste water by means of a second water tank 72. This tank 72 also nitrifies ammoniacal nitrogen and nitrite nitrogen to nitrate nitrogen. Subsequently, denitrification is executed by dissipating nitrogen into the atmospheric air in the form of nitrogen gas with the addition of excessive amount of adding organic matter (methanol or the like) that serves as a hydrogen donor by means of an organic matter tank pump 87 while introducing the waste water into a denitrification tank 89. Subsequently, the organic matter excessively added to the denitrification tank 89 is biologically treated by a re-aeration tank 91.
As prior art treatment equipment for treating nitrogen and fluorine containing waste water, there is the one disclosed in the prior art reference of Japanese Patent Laid-Open Publication No. 8-141597. This treatment equipment for treating the waste water containing nitrogen and fluorine is constructed of (1) a nitrifying unit of a biofilm type, (2) a defluorinating unit comprised of first and second reaction tanks and a sedimentation tank, (3) a biological denitrifying unit, (4) a biological oxidizing unit and (5) a sedimentation tank. In the present equipment, the waste water is first introduced into the denitrifying unit of the biofilm type filled with a microorganism immobilized carrier, and ammoniacal nitrogen is oxidized to nitrite nitrogen and nitrate nitrogen. The waste water is subsequently introduced into the defluorinating unit, and the fluorine in the waste water is removed to such a degree that no influence is exerted on the denitrifying bacteria. Subsequently, the waste water is introduced into the biological denitrifying unit with methanol added to the waste water, and the nitrogen becomes nitrogen gas as a consequence of the progress of the denitrification reaction until the denitrification is completed. Subsequently, the excessive methanol in the waste water is treated by the biological oxidizing unit. Then, the waste water is introduced into the sedimentation tank so as to be separated into supernatant liquid and sludge, and the sludge is sent back to the biological denitrifying unit.
The fluorine waste water discharged from the general semiconductor plant for manufacturing integrated circuits is mixed with chemicals containing nitrogen of nitric acid, ammonia water and ammonium fluoride as well as organic matter such as IPA (isopropyl alcohol) or acetone.
The nitric acid, ammonia water, ammonium fluoride IPA and acetone are sometimes singly discharged. However, through the semiconductor fabricating processes, hydrofluoric acid, nitric acid, ammonia water, ammonium fluoride, IPA, acetone and so on are generally used, and the chemicals are often handled in a clean bench of an identical process. Therefore, a small amount of those chemicals may enter a relatively large amount of hydrofluoric acid waste water.
As a method for treating the "fluorine waste water containing nitrogen and organic matter" that includes the mixture of nitrogen attributed to the nitric acid and ammonia water, the organic matter attributed to the IPA and acetone and fluorine, there is the one as follows.
The most generic method has the steps of firstly (1) chemically treating fluorine by a calcium preparation such as slaked lime and the coagulant of polychlorinated aluminum or the like, subsequently (2) biologically treating the organic matter concurrently with nitrifying the ammoniacal nitrogen and nitrite nitrogen into nitrate nitrogen, subsequently (3) biologically treating (denitrifying) the nitrate nitrogen with addition of methanol that serves as a hydrogen donor and then (4) biologically decomposing the methanol that has been excessively added in the preceding stage.
However, according to this fluorine treatment by the slaked lime, the intended fluorine concentration cannot be achieved unless the excessive amount of slaked lime and coagulant are added in order to reduce the fluorine concentration in the waste water to a single digit. As a result, unreacted slaked lime flows into the sedimentation tank to increase the amount of generation of sludge. This also leads to the problem that the amount of generation of sludge increases due to the excessive addition of the slaked lime, causing a cost increase.
Accordingly, as a method for reducing the amount of generation of sludge in treating the fluorine waste water, there has been developed the methods for removing the fluorine as calcium fluoride by means of calcium carbonate as adopted by the aforementioned pieces of waste water treatment equipment shown in FIG. 12 and FIG. 13. Further, as a further developed method, there has also been developed a method for removing the fluorine and organic matter in the waste water by utilizing chemical reaction and biotic reaction with microorganisms by means of calcium carbonate mineral as adopted by the aforementioned pieces of waste water treatment equipment shown in FIG. 14 and FIG. 15.
However, these methods, which have no aerobic tank where a sufficient anaerobic state exists, have the problem that the nitrogen cannot be treated although the fluorine and organic matter in the waste water can be treated.
As equipment for removing the fluorine, nitrogen and organic matter in the waste water, there is equipment for removing the fluorine waste water containing nitrogen and organic matter as shown in FIG. 16. However, this equipment for removing the fluorine waste water containing nitrogen and organic matter, which removes the organic matter in the waste water by the second water tank 72 that is the aerobic tank, is required to newly excessively add organic matter that serves as a hydrogen donor to the denitrification tank 89. Methanol can be enumerated as a concrete example of the organic matter. The methanol, which is excessively incorporated into the denitrification tank 89, is biologically treated in the subsequent re-aeration tank 91. Judging from the point of view of effective use of the resources, it is resources saving to effectively utilize the organic matter in the waste water as a hydrogen donor, and there is a demand for reasonable resources saving waste water treatment equipment. The nitrogen extraction ratio further improves when a stirrer 90 exists in the denitrification tank 89. However, if the stirring process exists, then the microorganic sludge having a degraded settleability flows from inside the tank, leading to a difficulty in greatly increasing the microorganic concentration. The reason why the microorganic concentration cannot be increased is that the intended microorganic sludge is not sufficiently generated due to the low concentration of organic matter in the fluorine waste water containing nitrogen and organic matter in the general semiconductor plant. In the case of the semiconductor plant waste water, the microorganic sludge is extinguished through aerobic digestion due to aeration even though the microorganic sludge is generated, so that the microorganic concentration cannot be increased. In the equipment shown in FIG. 16, the fluorine, organic matter and nitrogen in the waste water can be treated by installing the denitrification tank 89 and the re-aeration tank 91 in the stages subsequent to the treatment equipment of fluorine and organic matter. However, this method has the problems as follows. That is, the general semiconductor plants have a relatively large amount of fluorine waste water containing nitrogen and organic matter. Therefore, the amount of staying water becomes large assuming that the retention time in the denitrification tank 89 and the re-aeration tank 91 is several hours, and this leads to a vast amount of initial cost for the construction of tanks. Furthermore, a large installation area is necessitated. There is another problem that the organic matter that serves as a hydrogen donor attributed to the IPA and acetone in the waste water cannot be utilized since the organic matter is microorganically treated by the second water tank 72 that serves as an aerobic tank. Therefore, it is required to add an organic matter such as methanol that serves as a hydrogen donor to the denitrification tank 89. Further, the stirrer 90 for increasing the reaction efficiency is provided for the denitrification tank 89, and therefore, the anaerobic microorganic sludge having the degraded settleability cannot be maintained at high concentration. This also leads to the problem that the nitrogen extraction ratio during the denitrification is low.